CONTACTS OF INTRUSIVE MASSES 309 



i. e., the relation is like an inverted unconformity. This relation is 

 believed to be due to subsequent folding of the entire mass. The 

 process is illustrated in the three diagrams on page 307. (Figs. 

 5oa-5oc.) 



The end result is an intrusive mass which appears to cut across 

 folded strata and so is indistinguishable from the Chonoliths of 

 Daly. 



Igneous versus Sedimentary Contact. It is of the greatest im- 

 portance that the nature of the contact between igneous and non- 

 igneous formations be determined. Three types of contacts may 

 be recognized, i. Fault contacts, 2. Igneous contacts, and 3. Sedi- 

 mentary contacts. Fault contacts are secondary contacts and will 

 be considered in a subsequent section, as they have no direct bearing 

 on the history of the igneous mass. Igneous and sedimentary con- 

 tacts, on the other hand, are primary and intimately connected with 

 the origin of one or the other of the formations in contact. The 

 igneous contact stamps the igneous mass as the younger, while the 

 sedimentary contact shows the sedimentary formation to be the 

 younger of the two. As a rule, when exposures are sufficiently 

 good, the nature of the contact is not difficult to determine unless 

 both igneous and sedimentary mass have subsequently become meta- 

 morphosed. Sometimes the subsequent intrusion of igneous ma- 

 terial along the contact of an older igneous with a sedimentary for- 

 mation obscures the nature of the original contact. 



The chief criteria for use in determining relative ages of igneous 

 intrusive bodies in contact with sediments are the following : 



Contacts of subterranean or abyssal masses. 



(i) If the granitoid boss, stock or batholith is in contact with 

 sediments of known age, and these sediments have been metamor- 

 phosed or partly remelted by the igneous rock, which may even 

 enclose fragments of the sediment, then the igneous rock is of 

 younger age than the sediments, which were there before the molten 

 magma ate its way into them. 



(2) If sediments of a known age rest unconformably upon a 

 granitoid mass (granite, syenite, diorite, gabbro, etc.), worn frag- 

 ments of which are included in the sediments next adjoining the 

 igneous mass, the age of the igneous mass is very much greater 

 than that of the adjacent strata, for the igneous mass was exposed 

 by erosion before the sediments now resting upon it were laid down. 

 In such case the igneous mass has not metamorphosed the sedi- 



